GENERAL LAW OF NERVE EXCITEMENT. 



149 



may be most easily and surely shown in the case of 

 electric excitement, as there is no difficulty in allowing 

 the strength of the currents to increase or decrease 



O 



more or less gradually. Let the apparatus be arranged 

 as in fig. 35 in which the nerve is traversed by a 



FIG. 35. RHEOCIIORD. 



current, the strength of which may be altered by moving 

 the slide S. Let a key be inserted in the circle, and let 

 the slide be so placed that pulsations occur on the 

 closing and the opening of the current. On placing the 

 slide S close to A (in which position the resistance in 

 the branch A S is nil, so that no current passes through 

 the nerve), and pushing it slowly forward to its former 

 position at S, the current within the nerve slowly in- 

 creases from zero to its former strength : on again push- 

 ing the slide slowly back till it touches A, the strength 

 of the current again slowly decreases to 0. In neither 

 of these cases is the nerve excited. As soon, however, 

 as the movement of the slide is in any way effected 



1 E. du Bois-Reymond has described apparatus of this sort ur.der 

 the name of Schn-ankitnr/srhc'oehurd. 



